The proposed studies will characterize the diversity of the human antibody response to polysaccharide antigens using the capsular polysaccharide of H. influenzae type B (Hib) as an antigen. Hib is the most frequent cause of bacteremia and meningitis in children beyond the neonatal period. The polysaccharide capsule is thought to be its most important virulence factor and antibody to this antigen provides complete protection. The antibody response to this and other polysaccharide antigens is poor in infancy and remains highly variable in adults. Genes in or near the immunoglobulin heavy and light chain loci have been shown to be involved int he regulation of the antibody response to polysaccharide antigens and to influence the risk of disease in young children. In these studies the emphasis will be on examining the restricted nature of the antibodies recruited in the response to Hib, the genetic and regulatory mechanisms responsible for this restriction and the impact of this restriction on the magnitude and quality of the antibody produced during development and in adulthood. The technique of isoelectric focusing will be used to analyze the overall diversity of the human antibody to Hib capsular polysaccharide before and after immunization. Evidence for the presence of shared antibody clones (shared spectrotypes on isoelectric focusing gels) in unrelated individuals will be sought. The diversity of the antibody spectrotypes and the presence of shared spectrotypes will be analyzed in relation to (1) correlation with high or low responsiveness in adults (2) control by genes mapping immunoglobulin or major histocompatibility loci (3) change during ontogeny from infancy to adulthood and (4) influence of immunization with T-cell dependent forms of Hib polysaccharide. In the second phase of the studies we will prepare anti-idiotypic antibodies to human monoclonal antibodies to Hib or to shared spectrotypes utilizing rabbits and murine hybridomas. We will search for one or more cross-reactive idiotypes and analyze (1) how widely they are shared in immunized adults (2) whether they are correlated with high or low antibody responsiveness (3) whether they are present early in development and (4) whether their maturation can be accelerated by immunizing with T-cell dependent forms of polysaccharide. Future studies will look for autologous anti-idiotypes and determine whether they are important in regulating antibody responses to polysaccharides. There studies will enhance our understanding of the human immune response to polysaccharide antigens and may lead to new approaches in immunizing with these antigens.